This function is only valid when exporting using the UWP target for the Xbox One and it requires you to have checked the Enable XBox Live option in the UWP Game Options. The function will get the Gamer Tag for the currently signed-in user. Note that this function is only valid if a user is signed in and as such you should do a check for this using the function xboxlive_user_is_signed_in before requesting the Gamer Tag. If you call this function when no user is signed in you will get an empty string"" returned.
global.GamerTag = xboxlive_gamertag_for_user();
The above code checks to see if a user is signed-in to XBox Live and if they are signed-in, the code will get the gamertag for the user and store it in a global variable for future use. If they are not signed-in then the code checks to see if they are in the process of signing-in, in which case nothing further will happen, and if they are not, then it will open the account picker, prompting them to sign-in.